1 Field of the Invention
This invention relates generally to an oscillator and, more particularly, to an oscillator suitable for use in the horizontal oscillation circuit of a multi-scan television receiver.
2. Description of the Background
The so-called multi-scan television receiver must be capable of displaying signals that may include a video signal from a micro-computer output, as well as a standard video signal produced in accordance with one of the standard video formats. The problem is then presented that the horizontal frequency of the various signals may not be identical. More specifically, a video signal produced according to the NTSC system has a horizontal frequency of 15.75 kHz. On the other hand, the video signal from a micro-computer has a horizontal frequency of from 24 to 25 kHz. Another signal that is frequently displayed in a multi-scan type television receiver is a double-speed video signal, so that such double-speed video signal in the NTSC system will have a horizontal frequency of 31.5 kHz. In order to accommodate these various signals the horizontal oscillation circuit in the so-called multi-scan television receiver is designed so as to be capable of continuously changing the oscillation frequency in a range of from roughly 15.75 kHz to 31.5 kHz.
One known approach to achieving this variable oscillation frequency is to use the frequency of the input horizontal synchronizing pulse to derive a voltage and then to use that voltage as a control voltage to vary the oscillation frequency of the horizontal oscillator. Accordingly, it will be appreciated that a large number of adjustments are required in order to properly change the oscillation frequency so that it can closely follow the changes in frequency of the horizontal synchronizing pulses of the input signal. This then requires relatively complex circuitry that provides increased cost and decreased reliability to the multi-scan television receiver.